DESCRIPTION: The vernier acuity of 3-month-olds is 50-150 times worse than that of adults, whereas resolution acuity is only about 10 times worse. Vernier acuity is a measure of fine spatial localization that requires additional perceptual processing beyond the simple detection of the stimuli. The overall goal of this research project is to understand which differences between infants and adults are critical immaturities that limit infant vernier acuity and which are not. These immaturities may be divided into two groups: those that lead to poor visual sensitivity and those that are specific to spatial vision. Visual sensitivity. When vernier stimuli are designed to "level the playing field" by being equally visible to infants and adults, are infant and adult vernier acuities similar? In Experiments I, II, and III, stimuli will be created from Fourier components that are equally visible to infants and adults. This will be accomplished by applying a combination of blur, contrast reduction, and added "white" noise to the adult stimuli. If these manipulations bring adult vernier acuity into the infant range, this will suggest that infants' poor vernier acuity is due to their visual insensitivity. In Experiment IV, vernier acuity will be studied using a sinusoidal stimuli and a trigonometric analysis. If infant and adult vernier acuity can be predicted from their contrast discrimination performance, that will be evidence that visual insensitivity is responsible for infants' poor vernier acuity. This analysis will also be applied to adult amblyopes to find out whether strabismic and anisometropic amblyopia are caused by an arrest of visual development. Spatial vision. If visual insensitivity alone cannot explain infants' poor vernier acuity, what is the nature of the additional immaturities? In Experiment V, spatial uncertainty or scrambling of the retinotopic map between stages of visual processing will be compared by examining infant sensitivity to "white" luminance noise and positional noise added to the vernier stimulus. Positional noise is introduced by randomly jittering the spatial positions of pixels in the stimulus. If positional noise has a bigger effect on vernier acuity than luminance noise does, that will be evidence that spatial uncertainty is partly responsible for the poor vernier acuity of infants. In Experiment VI, the offset segments of the vernier stimulus will be separated by a gap. The critical value of the width of that gap will be compared to the amount of intrinsic blur from Experiment I to determine whether the infant's visual representation of the vernier stimulus is significantly undersampled. Clinical relevance. In the course of this basic-science project, the PI will find out whether normal infant vernier acuity is poor simply because of poor visual sensitivity or whether, like strabismic amblyopes, infants also have important immaturities in their spatial vision. If normal infants behave like normal adults then a vernier acuity task is likely to be useful for diagnosing amblyopia in infancy; if normal infants behave like adult strabismic amblyopes, a clinical test of vernier acuity is less likely to be useful.